1. Field of the Invention
This invention relates to the preparation of gasoline blending additives used to increase a transportation fuel's octane. More specifically, this invention relates to the use of strong acids, like sulfuric or hydrofluoric acids, to catalyze the addition of tertiary carbon atoms such as found in t-butane to an alkene such as found in 2-butane.
2. Description of the Prior Art
Many studies are reported on an acid catalyzed alkylation of alkenes by an alkane containing a tertiary carbon atom bound to hydrogen. In all of these studies, a hydride ion transfer from the tertiary carbon to an alkane-alkene intermediate is proposed. An example of these reactions is: ##STR1## The production of a new tertiary carbonium ion that adds to another alkene allegedly drives the reaction forward as alkylation products such as 2,2-dimethyl hexane and acid catalyzed rearrangements thereof are removed by separation from the reaction medium. To avoid side reactions, such as alkenes reacting with the alkane-alkene intermediate or themselves, excess alkane over alkene is used, often on the order of 20:1 weight percent alkane:alkene. The reaction temperature is usually kept low in the range of 10.degree.-20.degree. C.
Examples of such literature references are briefly cited below.
Kirk-Othmer Encyclopedia of Chemical Technology (Fourth Edition, Vol. 2, page 87) refers to three studies of strong acid catalysis of tertiary carbon addition to an alkene. These studies were: L. Schmerling, Journal of the American Chemical Society, Vol. 68, 275 (1946); F. G. Ciapetta, Industrial Engineering Chemistry, Vol. 37, 1210 (1945); and J. E. Hoffman & A. J. Schriescheim, Journal of the American Chemical Society, Vol. 84, 953-961 (1962).
J. W. Otvos, D. P. Stevenson, C. D. Wagner, and O. Beeck reported in the Journal of the American Chemical Society, Vol. 73, 5741 (1951) in an article entitled, "The Behavior of Isobutane in Concentrated Sulfuric Acid", that L. Schmerling's interpretation of the alkylation of alkenes by isobutane was essentially correct. They found: 1. no skeletal isomerization of either n-butane or isobutane; 2. there was no hydrogen exchange between molecules of n-butane (intermolecular) or between primary and secondary carbon atoms of a single n-butane (intramolecularly); 3. the tertiary hydrogen of t-butane does not exchange with acid protons of sulfuric acid, but the primary hydrogens do; 4. the tertiary hydrogens can be made to exchange by the addition of 0.1% of either isobutylene or 2-butane; 5. the apparent exchange rate between tertiary deuterated C.sup.13 labeled and C.sup.12 unlabeled isobutane revealed the rate of loss of tertiary D from the labeled isobutane matched the rate of increase in the amount of D in the unlabeled isobutane.
These results provided the basis for proposing the following reaction scheme: ##STR2##
These experimental observations and interpretations have lead to the universally accepted conclusion that scrambling of tertiary deuterium atoms of isobutane (also known as 2-methylpropane, tart-butane, isobutane, i- or t-butane) was due to a hydride ion transfer of a tertiary hydrogen of isobutane to a secondary or tertiary carbonium ion produced indirectly by protonation of an alkene or from an addition product intermediate that results from isobutylene adding to itself due to an acid catalysis.
In summary, the sulfuric acid catalyzed addition of isobutane to an olefin, such as propylene or butene, was believed to require a hydride ion transfer of the tertiary hydrogen of isobutane to an alkane-alkene intermediate containing a carbonium ion to produce another tertiary carbonium ion that would propagate the reaction by adding to another olefin.
Provided that the alkane, such as t-butane, is in large enough excess over the alkene, the formation of polymers or polyadducts can be suppressed and minimized.
In reaching this invention, the inventor began with the conviction that the above proposed mechanism could not be right. The reasoning was as follows. It is unlikely that simple thermal energy at temperatures under 50.degree. C. will be sufficient to polarize the C--H bond in tertiary butane merely by the approach of a secondary or even a tertiary carbonium ion so that the transition state: EQU (CH.sub.3).sub.3 C(+)--H(-)--(+)CHRR';
where R and R' are any alkyl group such as found in a typical alkylation adduct, is reached. As the transition state is approached, the hydrogen in the C--H bond allegedly becomes more and more negative. Since a hydrogen atom in a chemical bond with carbon is simply a very small proton nucleus surrounded by electrons that are primarily under the influence of carbon molecular orbitals, a more accurate picture of the hydrogen to carbon bond is a hydrogen nucleus imbedded in the electron cloud that surrounds the carbon atom.
Utility and Objects of the Invention
In light of the importance of producing blending components to increase the octane of transportation fuels such as diesel and gasoline, this invention discloses in part and in some of its embodiments ways to vary process variables for a conventional acid catalyzed alkylation of a hydrocarbyl specie containing at least one tertiary carbon to an alkene that improves yields and process performance, such as reducing the amount of an organic sludge and other environmentally undesirable side reactions that would otherwise be produced.